When I got my license two years ago I used letters from owners of boats I've sailed on. One was from a guy in the UK with whom I've sailed a lot, and the other was from Billy Ruff'n's owner, my wife. Neither needed to be "certified" and were readily accepted by the examiner.
To write the letter showing time underway on BR (which was the vast majority of my time) I used our passage log and plus estimates at the number of 4 hour day sails we had taken in various months. I was conservative in the estimates as I didn't have written documentation to back the estimates for all months. I think if your sea time calculations look reasonable, the examiners will be reasonable as well. Of course, there's no guarantee you'll end up with a "reasonable" examiner, so It's best to have as much back up as you can.
On the question of using letters vs the sea service form, I think the rules allow the examiner to accept a letter from an owner that isn't you. If you're applying time on a boat you own, I think you have to use the sea service form. I was advised by the school I attended to use the letters where ever possible as it's a lot easier and requires less detail on the days/months/years, etc. I'll attach a copy of one of the letters below. It's in the format recommended by the school where I tested.
Last edited by billyruffn; 01-12-2009 at 02:49 PM.